Method for installing load bearing piles utilizing a tool with blade means

ABSTRACT

A method and apparatus for installing a pile or load-bearing element into soft ground, wherein a non-percussive force, rather than a percussive force such as that applied by a hammer or a jack, is applied to the top of a hole-forming tool or pile ( 1 ) so as to push the hole-forming tool or pile ( 1 ) in a substantially continuous motion to a first depth into the ground, and wherein the hole-forming tool or pile ( 1 ) is then pushed in a non-percussive manner to a second depth while being simultaneously rotated. A cast-in-situ pile is formed in soft ground by pushing a hole-forming tool ( 1 ) provided with fins ( 3 ) at its base into the ground. Once the required level has been reached, the hole-forming tool ( 1 ) is rotated and concrete or grout is pumped through the body of the hole-forming tool so as concomitantly to help displace and replace the soil swept away by the fins ( 3 ). The hole-forming tool ( 1 ) is then withdrawn, and further concrete or grout is supplied so as to form a predictably-shaped pile with enhanced bearing capacity.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage of PCT/GB98/03419 filed Nov. 13,1998 and based upon the U.K. national application UK 9724024.6 filedNov. 13, 1997 under the International Convention.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for installing apile and/or a concrete or grout column in the ground, in particular byway of soil displacement.

BACKGROUND OF THE INVENTION

It is known to install load-bearing piles or columns by various methods.A first method involves hammering a preformed pile into the ground in aseries of discrete steps. This method can be effective, but there is arisk of causing damage to the pile or to the ground due to thediscontinuous hammering action. Furthermore, much noise and vibration iscaused by hammering. An alternative method is to use a jack to install apile or column made up of a number of discrete sections. A first sectionis pushed into the ground by the jack, which is then reset, and a secondsection is then welded or bonded to the top of the first section. Thejack is then activated again, and the process repeated until therequired depth has been reached. This method is low in efficiency sincethe jack needs to be retracted after completing each single stroke sothat the next element of the pile can be inserted, particularly since atypical stroke length is less than 50 cm.

A second known method is continuous flight auger piling, in which anauger with a continuous flight is caused to enter the ground by way ofrotation. Soil is excavated by way of the auger flights before or duringthe time that the auger is withdrawn from the ground. As the auger isbeing withdrawn, concrete is pumped through the stem of the auger to thetip, thereby leading to the formation of a load-bearing pile or column.Such a method is described in the present applicant's U.K. patentapplication no. 9515652.7, the disclosure of which is incorporated intothe present application by reference.

Alternatively, as disclosed for example in WO 95/12050, it is possibleto use an auger head which does not excavate soil, but instead displacesthe soil and compacts it into the surrounding ground. This has theadvantage that less spoil is generated, and can lead to bettermaintenance of ground integrity and greater density in the vicinity ofthe pile installation.

However, both these methods require that an auger or similar device bescrewed into the ground, which takes a relatively long time andgenerally means that specific combinations of torque and thrust must beapplied in accordance with the ground conditions in order to achievepenetration, and these can be difficult to achieve. Another disadvantagewith these screw piling methods is that the piling tool is subjected toa large degree of wear. Furthermore, we have found that there appears tobe an inverse relationship between the rotational torque required toproduce downward thrust by way of the pitch of the auger flights and the“crowd force” (i.e. the total force applied along the longitudinal axisof the auger during penetration) generated at the rig which can be usedto achieve penetration into the ground. Indeed, the crowd force initself is not sufficient to achieve the desired penetration,particularly when premature resistance to penetration, e.g. caused bythe presence of a subterranean stratum of granular material such asgravel, is experienced.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda method of installing a load-bearing pile or column in the around,wherein:

i) a hole-forming tool or pile having a longitudinal axis is pushed intothe ground, substantially in the direction of its longitudinal axis, ina substantially non-percussive manner to a first depth; and

ii) the hole forming tool or pile is then pushed further into theground, substantially in the direction of its longitudinal axis, in asubstantially non-percussive manner to a second depth while beingrotated about its longitudinal axis.

The term “non-percussive” is to be understood as the application of asubstantially continuous force over a relatively protracted period oftime, e.g. over a number of seconds or even minutes. This is to bedistinguished from percussive methods of piling, in which, for example,a weight is repeatedly dropped onto the top of a pile so as, in effect,to hammer the pile into the ground. In this case, most of the force isapplied over a relatively short period of time, e.g. fractions of asecond. Furthermore, the rate of change of applied force with respect totime will, in practical terms, generally have a discontinuity, asopposed to non-percussively applied force, which will tend to have arate of change with respect to time which is substantially continuous.

The present invention is of particular use when installing or formingpiles in soft ground overlying a granular stratum, such as gravel or thelike. The rotational motion of the hole-forming tool or pile helps toovercome premature resistance to penetration which may otherwise preventthe attainment of desired depths. Generally, the hole-forming tool orpile is pushed into the soft overlying soil until it reaches thegranular stratum, at which point the hole-forming tool or pile isadditionally rotated. Rotation combined with the pushing force issurprisingly effective in penetrating granular strata, especially withcertain hole-forming tool or pile tip geometries, and enables theresulting pile to be well-founded.

The rotation may be continuous rotation in either direction;alternatively or in addition, back and forth rotation may be used, whichrotation may be less than or more than one revolution. Where back andforth rotation is used, it has been found that an oscillation frequencyof around 1 Hz is effective in aiding penetration of granular strata,although frequencies of an order of magnitude higher or lower, e.g.around 10 Hz to 0.1 Hz, are also envisaged. In some applications, evenhigher or lower frequencies, e.g. around 100 Hz to 0.01 Hz, may beuseful.

This method may be used either to install a pile, such as a steel orpre-cast concrete pile, directly into the ground, or may be used toinsert a hole-forming tool, such as a hollow cylindrical tube providedwith a sacrificial end plate, into the ground so as to allow aload-bearing concrete or grout column to be cast-in-situ prior to orduring removal of the tool. The pile or tool dimensions and the forceapplied to the pile or tool are advantageously determined in accordancewith the ground conditions.

Preferably, the hole-forming tool or pile is pushed in a continuousmotion to a given depth of at least 1 m into the ground, and in someapplications, at least 2 m or even 5 m into the ground. Once the givendepth has been reached, the continuous force may be reapplied one ormore times so as to push the hole-forming tool or pile to a greatergiven depth, such as the depth of a granular stratum.

In this way, we have found that it is possible in certain groundconditions to attain depths in excess of 5 m in a time of around 16seconds, as opposed to 4 minutes using a rotating auger.

According to a second aspect of the present invention, there is provideda method of boring into the ground, wherein:

i) a hole-forming tool having a longitudinal axis is pushed into theground, substantially in the direction of its longitudinal axis, in asubstantially non-percussive manner to a first depth; and

ii.) the hole-forming tool is then pushed further into the ground,substantially in the direction of its longitudinal axis, in asubstantially non-percussive manner to a second depth while beingrotated about its longitudinal axis.

The hole-forming tool is generally withdrawn from the ground after ithas reached the required depth, although in some applications the toolmay be sacrificed and left in the ground.

In some embodiments, the hole-forming tool has a generally pointed tip,since this can be effective in penetrating granular strata when pushedinto the ground with concomitant rotation. However, a particularlypreferred embodiment utilises a hole-forming tool with a generally flatbase. We have found that upon pushing such a flat-based tool into theground, soil under the flat base tends to be compressed into a cone ofrelatively higher density than the surrounding soil. This cone ofrelatively high density soil substantially stays with the base of thehole-forming tool during penetration and rotation, and serves to disturbthe underlying soil in the desired manner. In some applications, it isadvantageous to spread a carpet of gravel or other granular material onthe top of the ground prior to penetration of the hole-forming tool. Theflat-based hole-forming tool is then lowered onto the carpet of gravelor other granular material and pushed into the ground, thereby using thegravel or granular material to make up at least some of the resultingcone. Although a proportion of the soil and/or gravel or other granularmaterial forming the cone of relatively high density may be lost to thesurrounding soil, this proportion is usually relatively low, and in anycase will generally be replaced by soil underlying the base of thehole-forming tool.

A pushing force is applied to the hole-forming tool in a similar mannerto that described in relation to the other aspects of the presentinvention, as is the rotation. Advantageously, the magnitude andduration of the applied non-percussive force, as well as those of thetorque applied to generate rotation, are monitored and controlled byelectronic computer means.

According to a third aspect of the present invention, there is provideda rig for inserting a hole-forming tool or pile having a longitudinalaxis into the ground, wherein the rig is provided with first means forapplying a substantially non-percussive force to the hole-forming toolor pile so as to push this into the ground, substantially in thedirection of its longitudinal axis, to a first depth, and wherein therig is further provided with second means for applying a rotation to thehole-forming tool or pile about its longitudinal axis, so as, incombination with said first means, to push the hole-forming tool orpile, substantially in the direction of its longitudinal axis, in asubstantially non-percussive manner to a second depth.

Preferably, the rig is adapted to push the hole-forming tool or pile ina continuous motion to a given depth of at least 1 m, and in someapplications, at least 2 m or even 5 m into the ground. Once the givendepth has been reached, the continuous force may be reapplied one ormore times so as to push the hole-forming tool or pile to a greatergiven depth, e.g. that of a granular stratum, before the hole-formingtool or pile is rotated.

One feature of the present invention is that the substantiallynon-percussive pushing force applied directly to the hole-forming toolor pile is greater than the downwards forces which may be generated byrotation of the hole-forming tool or pile. This is in complete contrastto screw piling methods, in which a substantial downwards force isgenerated due to the reaction between the helical rotating auger flightand the soil, especially in dense, cohesive soils. Advantageously, thedirectly-applied force is at least two, and preferably at least fivetimes greater in magnitude than the incidental downwards force which maybe generated by rotation.

By way of the present invention, it is possible to make use of a goodproportion of the weight of a typical piling rig, for example 70 tonnes,to help push. a hole-forming tool or pile into the ground. Given noconstraints on the piling rig abilities, we have found that it isrelatively straightforward to generate substantially only a downwardforce to achieve penetration of the hole-forming tool or pile. Thelimitations to this process are determined by the forces presented asresistance to penetration, and several fundamental factors need to beconsidered:

i) The resistance to penetration into the ground is dependent on thespecific ground conditions and soil type.

ii) the resistance is proportional to the cross-sectional area of theelement being inserted, and therefore the smaller the cross section, thelower the downward force required.

iii) The maximum downward force cannot exceed that available from thepiling rig used to install the hole-forming tool or pile.

An advantage of the present invention is that once the skin friction isovercome upon penetration, the resistance experienced by thehole-forming tool or pile during motion is often less than the staticresistance which may be encountered when the hole-forming tool or pilehas been left undisturbed for a period during which soil “set-up”occurs.

It may sometimes be advantageous to apply vibration to the hole-formingtool or pile so as to reduce skin friction in the event that penetrationis interrupted, but in any case the downward forces generated by suchvibration will be significantly less than those required to insert thehole-forming tool or pile to a greater depth. Vibration may also be usedas an aid to the expeller head should any obstructions impede itspenetration into the ground.

The non-percussive force applied by the rig to the top of thehole-forming tool or pile may be provided by way of a weight adjustablysuspended thereover. Alternatively or in addition, the rig may beprovided with a winch arrangement which is adapted to pull down the topof the hole-forming tool or pile.

In a particularly preferred embodiment, the rig is provided with ahydraulic ram with an extension of at least 1 m, and preferably at least2 m or in some embodiments at least 5 m. Such a ram may be used to applya non-percussive downwards force to the top of the hole-forming tool orpile so as to achieve depths of, respectively, at least 1 m, 2 m or 5 min a single operation. The ram may then be reset and arranged to reapplythe non-percussive force so as to achieve even greater depths.Additionally, the ram may be used in conjunction with a suspended weightand/or a winch arrangement and/or a vibrator.

Rotations may be applied by way of an electric, hydraulic or pneumaticmotor, or by any other suitable means, including manual means.

Where a hole-forming tool is pushed into the ground, concrete or groutmay be pumped from the surface so as to emerge at; or near the tip ofthe tool as the tool is rotated and/or withdrawn. In this way, acast-in-situ pile is formed. Advantageously, the volume of concrete orgrout being pumped along the length of the hole-forming tool ismonitored, for example by way of an electromagnetic flowmeter, andcontrolled by flow control means and electronic computer means. Theelectronic computer means additionally monitors and controls therotation and/or the extraction of the hole-forming tool. In this way, itis possible to control the rate of rotation and/or withdrawal of thehole-forming tool as a function of concrete or grout flow, or viceversa, so as to help ensure that the resulting cast-in-situ pile isstructurally sound.

According to a fourth aspect of the present invention, there is provideda method of forming an underground cast-in-situ pile, wherein:

i) a hole-forming tool comprising a body having a longitudinal axis anda lower end to which is fitted at least one blade-like attachment whichextends beyond the diameter of the body is pushed into the ground,substantially in the direction of its longitudinal axis, in asubstantially non-percussive manner to a first depth;

ii) at least the lower end of the hole-forming tool on which the atleast one blade-like attachment is mounted is rotated in such a way thatthe at least one blade-like attachment displaces soil at or close to thebase of the hole-forming tool while concrete or grout is concomitantlypumped along the length of the hole-forming tool so as to assist theblade-like attachments in the displacement of soil and to generate abase for the resulting pile; and

iii) the hole-forming tool is withdrawn while concrete or groutcontinues to be pumped along the length of the hole-forming tool.

Preferably, the hole-forming tool is pushed in a continuous motion to agiven depth of at least 1 m into the ground, and in some applications,at least 2 m or even 5 m into the ground. Once the given depth has beenreached, the continuous force may be reapplied one or more times so asto push the hole-forming tool to a greater given depth.

The hole-forming tool used in this aspect of the invention isparticularly suited to soft ground conditions, such as soft clayoverlying sands, silts overlying gravels and any generally soft materialoverlying granular material or suitable rock or bedrock, and is used toinstall a pile which can transfer load from ground level to stiffersoils at depth.

The hole-forming tool is pushed into the ground until its base reachesthe required depth, which will typically be the top of a loose to mediumdense sand or granular layer or rock or bedrock. A vibrator may beapplied to the top of the hole-forming tool in order to aid penetrationthrough any sand lenses or stiff layers or the like which may beencountered before the tip of the hole-forming tool reaches the requiredfounding level. The vibrator power may be of the order of 15 bhp,although any suitable power rating may be chosen in accordance withground conditions. It is to be noted that the vibrator is not the key topenetration of the tool, but may be provided merely as a secondary meansof achieving penetration into non-cohesive soils. The vibrator may alsobe used to assist with the extractions of the hole-forming tool.

Alternatively or in addition, the hole-forming tool may be rotated aboutits longitudinal axis without concrete or grout being pumped, whilebeing pushed into the ground in a substantially non-percussive manner.The rotation may be continuous rotation in a given direction, or may beback and forth rotation as described with reference to the first aspectof the present invention. This rotational movement, when combined with asubstantially non-percussive pushing force, allows the hole-forming toolto penetrate the layer or layers of sand or granular material. Therotational movement of the hole-forming tool results in the displacementof soil by the blade-like attachments. As a result of this displacement,ground pressure around the base of the hole-forming tool is lowered,thereby allowing material under the base of the hole-forming tool tomove upwards, thereby causing the ultimate end bearing to fail at a muchreduced loading.

Once at the required depth the hole-forming tool, or at least thesection of the hole-forming tool on which the at least one blade-likeattachment is mounted, is rotated in such a way that the at least oneblade-like attachment displaces soil at the base of the hole-formingtool, advantageously aided by the pressure of the concrete or groutpumped to the base of the hole-forming tool. As the soil is displaced,it is concomitantly replaced by concrete or grout which may be pumped atpositive pressure, for example up to 4 bar or even higher, through thebody of the hole-forming tool and emerge from at least one apertureprovided behind the at least one blade-like attachment or from at leastone aperture provided at or near the tip of the tool, thereby forming asubterranean bulb of concrete or grout: of predictable shape. It is tobe noted that as the hole-forming tool is rotated, soil in front of eachblade-like attachment is compressed, thereby increasing the local soilpressure relative to the surrounding soil pressure. Correspondingly,soil behind each blade-like attachment is allowed to expand, therebydecreasing the local soil pressure relative to the surrounding soilpressure. Alternatively, in embodiments where the body of thehole-forming tool is solid, concrete or grout may be pumped through aseparate feed pipe or pipes. Once the bulb has been formed, thehole-forming tool is extracted, preferably without rotation andpreferably in the same orientation as that used during insertion so asto keep resistance to extraction low. In some embodiments, the at leastone blade-like attachment may be arranged in a helical configurationabout the body of the hole-forming tool. This means that thehole-forming tool may be withdrawn with rotation, provided that the rateof withdrawal is controlled as a function of the pitch of the helixand/or the change of depth and/or the rate of rotation. Duringextraction, further concrete or grout is delivered so as to form acontinuous shaft up to a predetermined level. The delivery of concreteor grout during extraction must be sufficient to ensure that at least apredetermined minimum cross-section of shaft is cast. It is to be notedthat with displacement piling tools such as the hole-forming toolhereinbefore described, the soil surrounding the tool will tend to plugany escape path for concrete or grout; consequently, the positivepressure of the concrete or grout can generally be maintained. If thetool is rotated while the concrete or grout pressure is maintainedgenerally constant, then concrete or grout is absorbed into the areasurrounding the fins and the original soil is displaced untileventually, after several rotations, substantially all of the originalsoil is successfully displaced and replaced with concrete or grout. Thisis readily signalled by a significant drop in the torque required torotate the tool.

Advantageously, the volume of concrete or grout being pumped duringformation of the base and shaft of the cast-in-situ pile is monitored,for example by way of an electromagnetic flowmeter, and controlled byelectronic computer means. The electronic computer means may also beadapted to monitor and control the insertion and/or rotation and/orextraction of the hole-forming tool. For example, given the area of theat least one blade-like attachment and the rate of rotation, it ispossible to program the electronic computer means to calculate the rateat which the soil is displaced, and consequently to calculate the rateat which concrete or grout should be pumped so as to help ensure thatthe resulting load-bearing pile or column is structurally sound. Theelectronic computer means is advantageously adapted to control the rateat which concrete or grout is pumped and/or the rate of rotation of thehole-forming tool in accordance with these calculations. The electroniccomputer means may additionally be adapted to monitor and controlwithdrawal of the hole-forming tool and the flow of concrete or groutduring extraction so as to ensure that sufficient concrete or grout issupplied.

By displacing the soil at the base of the hole-forming tool andconcomitantly injecting concrete or grout, the base of a cast-in-situpile with enhanced bearing capacity is formed. The enhanced bearingcapacity is a result of the increased diameter of the base of the pilerelative to the shaft of the pile. The diameter of the base may beseveral times the diameter of the shaft.

The number of blade-like attachments mounted on the hole-forming tool ispreferably two, for reasons of symmetry, but other numbers ofattachments may be just as effective. The minimum number of rotationsrequired to achieve the necessary soil displacement is 1/(number ofattachments), provided that the attachments are equiangularly spacedabout the circumference of the hole-forming tool, although depending onground conditions, a greater number of rotations may be required so asto ensure that the soil is fully displaced. In some embodiments, thehole-forming tool is rotated first in one direction, and then in theother direction. The rate of rotation is advantageously controlled sothat the volume of soil displaced is less than, or at least equal to,the volume of concrete concomitantly supplied.

In a particularly preferred embodiment, the volume of soil displaced isbetween 50 and 500 litres (0.05 to 0.5 m³), which is relativelyefficient in terms of material usage. Additional rotations andadditional concrete or grout delivery may also be used in order toincrease the size of the bulb, but this may not be of great advantagesince the final shape of the bulb may no longer be as predictable andthe effective area of the bulb may not increase.

Conventional drive means, such as a motor or even manual means, may beemployed to rotate the hole-forming tool. Alternatively, one or morehydraulic rams may be used to rotate the hole-forming tool in astep-wise manner. The one or more hydraulic rams are adapted to engagewith arms projecting axially from the body of the hole-forming tool,thereby providing step-wise rotation with a force great enough to ensurethat the blade-like attachments displace the soil surrounding the lowerend of the hole-forming tool. In a hole-forming tool with twodiametrically opposed blade-like attachments, rotation through at least180° is required, whereas in a hole-forming tool with fourequally-spaced blade-like e attachments, rotation need only be throughat least 90°.

The blade-like attachments are preferably sized and shaped so as not todeform excessively upon rotation. The blade-like attachments may besolid, or they may be provided with holes or apertures which enable theconcrete or grout to be mixed in-situ with soil materials.Alternatively, each blade-like attachment may comprise severalprotruding elements separated by slits. The precise dimensions andshapes of the blade-like attachments may be selected in accordance withthe ground conditions. In some embodiments, it is envisaged that theblade-like attachments may be disposed at an angle to the longitudinalaxis of the hole-forming tool, thereby tending to displace soil upwardsor downwards upon rotation.

The enlarged base-forming technique described above may be adapted so asto provide an extended enlarged base. This may be done by continuing torotate the tool while extracting it slowly through the founding stratumand concomitantly supplying concrete or grout. Once the top of thefounding stratum is reached, rotation of the tool is stopped, andwithdrawal from the ground completed in the normal way. This results ina pile having an elongate base of wider diameter than the main body ofthe pile, which may have enhanced skin friction as well as enhanced endbearing capacity.

It is also possible for the enlarged base formation technique to berepeated at several depths within the founding stratum, or even tocontinue rotation while extracting the tool at a rate which allows acontinuous spiral base enlargement to be produced.

A further possibility is to apply a degree of back and forth rotationduring extraction of the tool. In this way, a pile having a central bodyis provided with “wings” defined by the width of the blade-likeattachments, which may have enhanced skin effects due to the resultingpile having a greater circumferential surface area than that of a simplecylinder.

The cast-in-situ pile may be provided with an enlarged head at or belowthe surface of the ground by way of additional rotation of the tool andconcomitant additional concrete or grout delivery once the hole-formingtool has been withdrawn to a predetermined level. This enlarged head naybe provided just below the ground surface or at greater depths if areduction of ground level is expected. In some applications, theenlarged head may be formed in a carpet of gravel or other appropriategranular material which has been provided at the surface level.

Once the pile has been cast, a reinforcement may be inserted. This maytake the form of a single or multiple steel bar arrangement which ispushed to a predetermined depth into the wet concrete or grout beforethis has set.

BRIEF DESCRIPTION OF THE DRAWING

For a better understanding of the present invention and to show how itmay be carried into effect, reference will now be made, by way ofexample, to the accompanying drawings in which:

FIG. 1 shows a hole-forming tool of the present invention after it hasbeen pushed into the ground;

FIG. 2 shows in cross-section a first arrangement of blade-likeattachments on the hole-forming tool of FIG. 1;

FIG. 3 shows in cross-section a second arrangement of blade-likeattachments on the hole-forming tool of FIG. 1;

FIG. 4 shows in cross-section a third arrangement of blade-likeattachments on the hole-forming tool of FIG. 1;

FIG. 5 shows a rig fitted with a hole-forming tool and a suspendedweight;

FIG. 6 shows a rig fitted with a hole-forming tool and a hydraulic ram;

FIG. 7 shows a hole-forming tool in schematic form;

FIGS. 8 and 9 show the hole-forming tool of FIG. 7 in use;

FIG. 10 shows an alternative embodiment of a hole-forming tool of thepresent invention;

FIG. 11 shows a pile formed by the tool of FIG. 10; and

FIGS. 12 to 16 show alternative piles formed by the tool of FIG. 10.

SPECIFIC DESCRIPTION

The hole-forming tool 1 shown in FIG. 1 comprises a body 2 provided withtwo blade-like attachments of fins 3 at or near its base. The diameterof the body 1 of the embodiment shown is 0.3 m, and the maximum distancebetween the extremities of the fins 3 is 0.8 m. The fins 3 are shapedand sized so as to displace a volume of approximately 100 liters whenthe hole-forming tool 1 is rotated. Furthermore, the fins 3 are providedwith slanting edges so as to facilitate insertion and extraction of thehole-forming tool 1. The body 2 of the hole-forming tool 1 is hollow,and allows concrete or grout to be pumped from the top of thehole-forming tool 1 and out through a port 4 provided near the base ofthe hole-forming tool 1, although in other embodiments a separate groutor concrete feed pipe or pipes may be used.

In use, the hole-forming tool 1 is pushed into soft ground, such as clay5, in a continuous motion until the base of the hole-forming tool 1reaches the top of a layer of granular material, rock or bedrock 6. Arotational motion is then applied to the tool, and it is pushed furtherinto the ground. The rotational movement helps the tool to penetrateinto the granular material by decreasing the ground pressure around thetip of the tool and allowing material beneath the tip of the tool tomigrate upwards into the regions of decreased ground pressure. Avibrator 7 may be mounted to the top of the hole-forming tool 1 in orderto aid penetration through any intermediate sand lenses or stiff layers(not shown), as well as to reduce friction and to ensure entry of thebase of the hole-forming tool 1 into the sand layler 6. Typically, theforce required to insert the hole-forming tool of FIG. 1 will be lessthan 100 kN with an angle of friction of the soil, say of 30°, for thebasal sand.

Once the hole-forming tool 1 has been inserted to the required depth, itis rotated so that the fins 3 displace a volume of soil. At the sametime, concrete or grout is pumped through the body 2 of the hole-formingtool 1 arid out through the port 4, thereby forming a subterranean bulbof concrete or grout. The hole-forming tool 1 is then withdrawn withoutrotation while concrete or grout continues to be pumped so as to formthe shaft of a cast-in-situ pile.

The fins 3 may take various configurations, examples of which are shownin FIGS. 2, 3 and 4. The configurations of FIG. 2 and 3 are designed forrotation in one direction only, whereas the configuration of FIG. 4 maybe rotated in both directions.

The fins 3 need only be thick enough so as not to deform excessivelyupon rotation. In the illustrated embodiment, no permanent deformationof the fins 3 will occur in clays of up to a clay shear strengthtypically of 50 kN/mm² (undrained). In the arrangement shown, the fins 3span a distance of 0.8 m, thereby increasing the bearing surface area byseven times in relation to the diameter of the hole-forming tool 1,which is 0.3 m.

FIG. 5 shows a rig 7 on which is mounted a hole-forming tool 8. At thetop of the hole-forming tool 8, there is provided a drive unit 9 torotate the tool, and a vibrator unit 10. A weight 11 is adjustablysuspended over the top of the hole-forming tool 8 such that a downwardsforce can be applied to the top of the tool in order to push this intothe ground in a continuous motion. Instead of or in addition to theweight 11, there may be provided a hydraulic ram 11′ as shown i n FIG.6, which has an extension of at least 1 m, and typically at least 2 m oreven 5 m. The ram 11′ is used to push the hole-forming tool 8 to a givendepth of at least 1 m in a single continuous operation. The ram 11′ canthen be reset and used to push the hole-forming tool 8 to an evengreater depth. There may also be provided a winch arrangement 20 whichcan be used to pull down the top of the hole-forming tool 8. A concreteor grout feed pipe 12 is located at the top of the hole-forming tool 8so as to allow concrete or grout to be pumped through the body of thetool.

FIG. 7 shows a hole-forming tool 8 similar to that shown in FIGS. 5 and6. The hole-forming tool 8 has a head portion 13 and a body portion 14,which may be of variable length so as to be adaptable to differentground conditions. The head portion 13 and the body portion areconnected by means of a standard connector 15. Two fins 16 are providedtowards the lower end of the head portion 13. At the upper end of thebody portion 14, where the hole-forming tool 8 is supported by the rig7, there is provided a vibrator unit 10 and a drive unit 9. The driveunit 9 comprises a ram 17 attached to arms 18 in such a way thatactuation of the ram 17 will cause the hole-forming tool 8 to rotatethrough approximately 90°. A weight 11 is lowered onto the top of thehole-forming tool 8 so as to provide the downwards force required topush the tool into the ground.

FIG. 8 shows the head portion 13 of the hole-forming tool 8 of FIG. 7which has been pushed through clay 5 until the tip of the tool hasreached the top of a layer of medium dense sand 6. A concrete or groutdelivery nozzle 18 is provided at the lower end of the hole-forming tool8, the nozzle 18 being fitted with a reinforced bung 19 so as to preventingress of soil as the hole-forming tool 8 is pushed into the ground.

In order to form a pile or load-bearing column, concrete or grout ispumped through the body of the hole-forming tool 8, initially to pushout the bung 19. The hole-forming tool is then lifted by approximately100 mm and concrete or grout is pumped through the nozzle 18 at acontrolled rate. The hole-forming tool 8 is then rotated through 180°,as shown best in FIG. 9, such that the fins 16 displace a volume ofsoil, aided by the pumping of concrete or grout which is concomitantlypumped from the nozzle 18 at a controlled rate. Rotation is thenstopped, and the hole-forming tool 8 is withdrawn as concrete or groutcontinues to be pumped at a rate determined by the rate of withdrawal ofthe hole-forming tool 8, thereby forming a load-bearing column with anenlarged base, and hence increased bearing capacity. Further enlargedportions may be formed at other points along the length of theload-bearing column by interrupting the withdrawal of the hole-formingtool 8 with further periods of rotation.

An alternative design of hole-forming tool 21 is shown in FIG. 10. Thetool 21 is provided with two fins 22, and has a hollow stem 23 throughwhich concrete or grout may be pumped. Ports 24 are provided behind thefins 22 so as to allow concrete or grout to be output when required. Thetool 21 is used in the same manner as the tool 8 of FIGS. 7 to 9, butinstead of forming an enlarged pile base on top of a stratum of granularmaterial, the tool 21 is pushed into such a stratum by way of theadditional application of rotation. Once the tool 21 has, reached thedesired depth, it is then rotated while concrete or grout is pumpedthrough the hollow stem and the ports 4 as described above, before beingextracted with no rotation, thereby forming a pile 25 with an enlargedbase 26 in a granular stratum 27 as shown in FIG. 11.

Referring now to FIG. 12, a pile 28 with an elongate cylindricalenlarged base 29 may be formed by continuing to rotate the tool 21slowly during withdrawal from the granular stratum 27, while continuingto supply concrete or grout. Rotation is then ceased, and the tool 21withdrawn as before so as to form the main shaft of the pile 28. Theelongate cylindrical enlarged base 29 has improved skin frictioncompared to the simple enlarged base 26 of. FIG. 11.

It is also possible, as shown in FIG. 13, to form a pile 30 withmultiple enlargements 31. This is achieved by rotating the tool 21 at abase depth while supplying concrete or grout, then ceasing rotation andwithdrawing the tool 21 to a higher level before repeating the rotationat that higher level and further levels thereabove. If the tool 21 isrotated continuously while being extracted at a relatively fast rate, orrotated relatively slowly during withdrawal, a pile 32 with a spiralenlargement 33 may be formed, as shown in FIG. 14. The piles 30 and 32of FIGS. 13 and 14 have increased skin friction and potentiallyincreased end bearing capacity.

If the tool 21 is given a slight oscillatory rotation during withdrawal,a pile 34 with “wings” 35 may be formed, as shown in section in FIG. 15.These “wings” 35 may extend for the whole length of the pile 34, or maybe formed only on sections of the pile length. The increased surfacearea of such a pile 34 may provide improved skin friction. Furthermore,a. number of such piles 34 may be provided in a row so as to build asubterranean wall 36, as shown in FIG. 16.

What is claimed is:
 1. A method of forming an underground cast-in-situpile, with no extraction of material, using a hollow pile-forming toolcomprising a body having a longitudinal axis, aperture means in theregion of its lower end, and attached to its lower end, blade meanswhich extend beyond the diameter of the body, the method comprising: a)pushing the tool non-rotatorily into the ground substantially in thedirection of its longitudinal axis, in a substantially non-percussivemanner to a first depth; b) rotating the tool such that the blade meansdisplace soil at or close to the base of the tool, and concomitantlypumping concrete or grout along the length of the tool and through theaperture means so as to assist the blade means in the displacement ofsoil and generate an enlarged base for the resulting pile; and c)withdrawing the tool while concrete or grout continues to be pumpedalong the length of the tool.
 2. A method according to claim 1 whereinthe pushing is by means of a weight which is adjustably suspendible overthe tool.
 3. A method according to claim 1 wherein the pushing is bymeans of a hydraulic ram.
 4. A method according to claim 1 wherein thepushing is by means of a winch.
 5. A method according to claim 1 whereinthe tool is rotated in a single direction.
 6. A method according toclaim 1 wherein the tool is rotated back and forth.
 7. A methodaccording to claim 6 wherein a vibrator is used to vibrate the toolabout its longitudinal axis.
 8. A method according to claim 1 whereinthe force directly applied to the top of the tool is greater than anydownwards force which may result from rotation of the tool.
 9. A methodaccording to claim 8 wherein the force directly applied to the top ofthe tool is at least twice any downwards force which may result fromrotation of the tool.
 10. A method according to claim 1 wherein thevolume of the concrete or grout supplied is at least equal to the volumeof soil displaced by the blade means in the step (b).
 11. A methodaccording to claim 1 wherein the concrete or grout is supplied by way ofan electronic flowmeter and flow control means.
 12. A method accordingto claim 1 wherein an additional rotation of the tool and concomitantadditional concrete or grout delivery is performed once the tool hasbeen substantially fully withdrawn to thereby provide the case-in-situpile with an enlarged head at or close to the ground surface.
 13. Amethod according to claim 1 wherein the pushing of the tool, therotation of the tool, the rate of withdrawal of the tool, and theconcrete or grout flow rate are monitored and controlled, individuallyor collectively, by an electronic computer.
 14. A method of installing aload-bearing pile or column in the ground, wherein: a) a hole-formingtool or pile comprising a body having a longitudinal axis and a lowerend having blade means attached thereto, the blade means extendingbeyond the diameter of the body, the tool or pile is pushed into theground, substantially in the direction of its longitudinal axis, in asubstantially non-percussive and non-rotatory manner to an intermediatedepth without extraction of material; and b) the tool or pile is thenpushed further into the ground, substantially in the direction of itslongitudinal axis, in a substantially non-percussive manner while beingrotated about its longitudinal axis in such a way that the blade meansdisplace soil at or close to the base of the tool or pile.
 15. A methodaccording to claim 14 wherein the tool or pile is pushed by means of aweight which is adjustably suspendible over the tool or pile.
 16. Amethod according to claim 14 wherein the tool or pile is pushed by meansof a hydraulic ram.
 17. A method according to claim 14 wherein the toolor pile is pushed by means of a winch.
 18. A method according to claim14 wherein the tool or pile is rotated in a single direction.
 19. Amethod according to claim 14 wherein the tool or pile is rotated backand forth.
 20. A method according to claim 19 wherein a vibrator is usedto vibrate the tool or pile about its longitudinal axis.
 21. A methodaccording to claim 14 wherein the force directly applied to the top ofthe tool or pile is greater than any downwards force which may resultfrom rotation of the tool or pile.
 22. A method according to claim 21wherein the force directly applied to the top of the tool or pile is atleast twice any downwards force which may result from rotation of thetool or pile.
 23. A method according to claim 14 and as the hole-formingtool, wherein, after the step (b), the tool is withdrawn, and concreteor grout is pumped at positive pressure along the length of the tool andout of a nozzle located at or near the lower end of the tool duringwithdrawal so as to form a cast-in-situ pile.